1. Field of the Invention
The present invention relates to a device for adjusting thread tension of the doup ends and of the stationary threads that are guided through a leno selvedge forming device.
2. Description of the Prior Art
Such a leno selvedge forming device may for example be embodied by a combination of lifting healds and half healds. As it is known, two lifting healds connected to two half healds hereby serve to produce a so-called leno selvedge. Such lifting healds are directly or indirectly fixed to the heald frames of a loom. The doup end is guided between half heald and lifting heald whereas the stationary thread is threaded through the eyelet of the half heald. Due to the alternate motion of the doup end from one side of the half heald to the other side of the half heald, the weft is tied together with the stationary thread by the thus created leno selvedge. The leno selvedge prevents the fabric from fluting this area.
A so-called holding-down appliance for the thread is located directly before such a leno selvedge forming device consisting of two lifting healds and one half heald, that is directly before the heald frames, said holding-down appliance for the thread always keeping the dour end and the stationary thread under tension as they are guided through the lifting heald and through the half heald respectively. This is necessary since only by holding down the doup end and the stationary thread respectively it is made certain that the doup end is capable of reliably wandering at each weft change from one side of the half heald to the other side of the half heald even on fast-running power looms.
Now as it is, when the shed is being opened, the doup end and the stationary thread respectively are submitted to tension due to the lengthening of the path. In order to keep this tension low, a spring assembly is provided in the area of the bobbins that compensates the lengthening of the doup end and of the stationary thread respectively. On their way from the lifting healds to the bobbin, the stationary thread as well as the doup end are deflected several times, whereas it turned out that, due to the many deflections and to the thus originated friction losses, the spring assembly accommodated on the bobbin no longer compensates the tension of the thread on the bobbin. This means that the lengthening of the thread that occurs when the shed is opened is substantially provided by the elasticity inherent to the thread. One disadvantage thereof is that the threads are subjected to high strain; another substantial drawback is that, since, when the shed is opened, the threads are subjected to strain and thread tension increases accordingly, the threads that are passing alongside the lifting healds are cutting in more because or the increased tension than they would if the tension remained lower. That means that the lifting healds wear more.
FR 15 55 223 discloses a similar device. Here, a heald is arranged before a leno selvedge forming device, whereas the doup end's tension is adjusted in front of the heald. A spring-loaded lever s provided to this end, whereas the ever is fitted at its end with the eyelet for the doup end. The disadvantage thereof is that the doup end is always stretched when the shed is opened so that the thread incurs the risk of tearing or cutting in in the leno selvedge forming device. This is due to the fact that the device in question can only restrictedly proceed to thread tension adjustment because of the heald that is arranged in front of the leno selvedge forming device.